ar10 upper changes 7mm choices

[cgmaster1]

I have been thinking of modifying one of my uppers for my Armalite AR10. Right now I have a 243 t model upper and a standard a4 rifle 308 upper with chrome lined barrel.

I am thinking of changing one to a 7mm. I havent decided on which exact cartidge. If I go with a magnum cartridge I know I would have to modify the bolt face. I will want to keep a lighter weight barrel about 20" possibly 18". I have been investigating the bullet choices and it seems like 7mm is the best for long range work. I have thought about the 284 improved, 7mm/08, 7mm rsaum and 284 Shehane. I know there may be others out there and I will be looking into it further.

Has anyone here used 7mm through the ar10? Does anyone have any specifics on gas port size and location for the different cartidges? I am a little worried about getting the proper cartridge oal using the mag and having to crimp the bullet into the case. I would like the magnum cases to be able to push the bullet faster. Any other help would be greatly appreciated. Thanks In advance.

 

[Glen Seekins]

Re: ar10 upper changes 7mm choices

A 6mm will beat most everything out to 1k because you can get some speed out of them and they have a great BC. Unless you can push the big 7mm bullets at 3k that is

A .243 would be very hard to beat. If a 243ai fed well (i will know this summer) and fit in the mag well you wont beat it with anything else you can get to run

 

[bohem]

Re: ar10 upper changes 7mm choices

For that platform you're going to be hard pressed to get the 284 cases to work out well for you. While it is designated a SA round, it's really a bastard size like the 6.5 Swede. The SAUM cases would give you more boiler room by a fair bit but running that kind of powder capacity in an 18-20" upper is going to mean lots of sound/smoke/hell but performance will suffer, there just isn't enough barrel there to make the case perform like it can.

I ran some expectations of a 7mm-08 case against a 260 case in a 20" version and spent a little time with the JBM site comparing them.

The 260 wins by just a little each time from 120-140 bullets against 140-162gr bullets, I didn't try anything heavier from the 7's because of the barrel length.

The 243 (as well mentioned) is going to be the flattest round out there, however for downrange whomp, if you wanted to hunt with it, the 260 works out to be the best balance of bullet mass, speed, and BC.

After working through various combinations my buddy that asked settled on an LR260 upper, after shooting it I bought one a few weeks later.

 

[longshotbml]

Re: ar10 upper changes 7mm choices

For pure ballistic efficiency; 260 Remington. The BC of the available 6.5mm bullets is awesome.

 

[bohem]

Re: ar10 upper changes 7mm choices

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Longshotbml</div><div class="ubbcode-body">For pure ballistic efficiency; 260 Remington. The BC of the available 6.5mm bullets is awesome. </div></div>

Look at the same "heavy for caliber" 7's and you'll see why he wants a 7mm.

The shortcoming with the 7 is that the 308 case doesn't have the capacity to really launch the heavy 7's like you can do with a SAUM case (30-06 capacity) yet keeping the barrel short 18-20" for mobility, the SAUM case is crippled.

Hence my advice on the 260.

 

[bignada]

Re: ar10 upper changes 7mm choices

How about a .284win?

.473" case head, so no need for a special bolt.
Measures .500 on casebody so standard mag will hold more rounds and function with minor improvement of feed lips.

Heavy bullets are likely too long for best performance, but TANSTAFL. Think the Hornady 162gr bt would be a good performer.


Not really much to compromise with. The .308win is the optimum for the AR-10. If you want BC, might consider the .338-284 or the .358win running the Sierra 225gr bt. I have almost ordered an Armalite 10T upper in .338-08 a couple times, but the round is too anemic. The .284 case is The Ticket to AR-10 performance. Instead of the .300saum, would go .308-284...

If you want BC performance and barrel life, the .338-284 is worth evaluation.

 

[hopps]

Re: ar10 upper changes 7mm choices

it can be done but I am told getting the .284 to feed right can take a trick or two.

7mm-saum has some potential but it could potentially burn barrels and to be completely honest the .260/6.5creedmore remain the best balance of ballistic efficiency. 7mm carries a little more ft/lbs downrange but if you want ft/lbs at long range just shoot 208s or 210 grain bullets in .308

On locations of gas ports you might stop in at http://ar10t.com. There are some discussions there about optimum gas port placement when not going with standard .308 platform.

I think you will end up going to at least 250grain bullets in .338 or .358 to beat 6.5 or .308 bc offerings.

 

[Casey Simpson]

Re: ar10 upper changes 7mm choices

Lee Wells at Accurate and Reliable Rifles builds 6.5x47 uppers/barrels. I chose him because he's not running a cookie factory assembly line shop, guarantees his work 100%, and this round because I have two years experience shooting it (+1000rds?), its bullets, powder, primers, etc., and have brass, bullets, dies, etc.. The round is good.

But it is also a good choice due to its COAL. The 123 Scenar shoots well for me at 2.7"oal. Varget 38grs runs @2925fpsmv. The DPMS mags allow only about 2.8oal to be loaded in them in my experience. Scenars flat shoot but so do Norma and Berger 130's. They to will run in the 2.8 mag. Varget and H4350 are what burn and either, I hope, will be fine, but I will try for giggles RL17. Bush the bolt face, run a carrier weight, or a gas tuner block and the gun should shoot fine, or better than most of us.
BTW, I already had a 6.5 Krieger too so that influenced my choice a little bit too.

 

[hopps]

Re: ar10 upper changes 7mm choices

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Lee Wells at Accurate and Reliable Rifles builds 6.5x47 uppers/barrels.</div></div>

Awesome, I have had some questions about the 6.5 lapua in an ar platform but never had anyone to ask.

and

+1 on the adjustable gas block, big help when playing with loads imo...

 

[bohem]

Re: ar10 upper changes 7mm choices

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bignada</div><div class="ubbcode-body">

If you want BC performance and barrel life, the .338-284 is worth evaluation. </div></div>

Try running realistic numbers through a calculator like JBM with Bryan Litz's test data and you will soon realize that the heavy 7's have high BCs (and the 180 berger is better than the 250 SMK).

It will make very fast sense that your recommendation way off base.

If you really don't believe me, I have a couple ballistic plots I can send you to visualize it better.

 

[bustin]

Re: ar10 upper changes 7mm choices

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: cgmaster1</div><div class="ubbcode-body">I have been thinking of modifying one of my uppers for my Armalite AR10. Right now I have a 243 t model upper and a standard a4 rifle 308 upper with chrome lined barrel.

I am thinking of changing one to a 7mm. I havent decided on which exact cartidge. If I go with a magnum cartridge I know I would have to modify the bolt face. I will want to keep a lighter weight barrel about 20" possibly 18". I have been investigating the bullet choices and it seems like 7mm is the best for long range work. I have thought about the 284 improved, 7mm/08, 7mm rsaum and 284 Shehane. I know there may be others out there and I will be looking into it further.

Has anyone here used 7mm through the ar10? Does anyone have any specifics on gas port size and location for the different cartidges? I am a little worried about getting the proper cartridge oal using the mag and having to crimp the bullet into the case. I would like the magnum cases to be able to push the bullet faster. Any other help would be greatly appreciated. Thanks In advance. </div></div>
You say 20" maybe 18" and long range work, what kind of long range? like others have said a big magnum needs some barrel length to get the vel up, A larger bore is more efficient if you can run similar BC bullets as fast as the light small cal 6mms. Are you talking about hunting or punching paper?

 

[Heman]

Re: ar10 upper changes 7mm choices

The 7mm 175gr SMK is a great bullet. BC is actually about 6% higher than listed, this was confirmed with Bryan Litz. The problem with the 7mm-08 is your not going to have enough boiler room to push it. I built a 280 Rem for NRA Long Range and I can push that 7mm to shoot inside a 300 Win Mag all day long. The 7mm-08 I think would be able to reasonably push the 168gr SMK it's BC is actually 14% higher than what sierra lists. I have some serious thought to a 7mm-08 upper but then went the .260 rout 3 years ago and was very happy with my decision.